Echo in telecommunications systems may arise from impedance matches at a four-wire to two-wire hybrid interface, or from an acoustic feedback path at one end of a call. Echo cancellers are essential to long-distance telephonic communication in order to obtain intelligible full-duplex calls. An echo canceller operates by developing an impulse response model of the echo path, and then generating and subtracting an estimated echo signal from an outbound signal on the basis of the impulse response model and the inbound signal. Because an echo canceller alters the outbound telecommunication signal, it is desirable to disable echo cancellation for certain calls, such as data communication calls, and to enable echo cancellation for voice communication calls. One such method of selectively enabling or disabling echo cancellation is to transmit standardized control tones along the telecommunication path. Echo cancellers responsive to such control tones then enable or disable echo cancellation accordingly. In a time-division multiplexed communication system, a single echo canceller module may provide echo cancellation for several channels on a time-division processing basis. Hence, in addition to developing an impulse response model for each of the channels in which echo cancellation is enabled, a multi-channel echo canceller also maintains state information for each channel, indicated whether echo cancellation is enabled.
During the course of operation of a telecommunication system, it may be necessary to take an echo canceller out of active service. For example, an echo canceller may fail, or it may be desirable to upgrade or otherwise service an echo canceller. Because active telephone calls may be in the course of transmission at the time an echo canceller is taken out of service, it is desirable to provide a method in which an active echo canceller may be replaced by another echo canceller without interrupting call transmission. In the case of a multi-channel echo canceller, it would also be desirable for the replacement echo canceller to obtain the current state information indicating the status of echo cancellation for each channel.